Since the capacity and storage space varies from car to car, the general rule of thumb is to put the heaviest things as far forward and as close to the floor in the cargo area as possible. The goal is to keep the center of gravity low, reduce the weight over the wheels and prevent the car from becoming top-heavy.
In a car without a trunk, lighter items stored on top of the bulkier baggage should be secured so they don't fly forward in the case of a sudden stop. And don't pile your things to the ceiling. The driver should be able to see out of the rear window. Keep essential items (diapers, coats, change of clothes) within reach, and don't forget to take advantage of spaces under the seats [source: ConsumerReports.org].
Positioning people can be just as crucial. Passengers who are prone to carsickness need to be able to see out of the front window; and seat belt laws dictate who can sit in the front seat, as well as where to place car seats. Beyond that, consider who gets along best and who needs special assistance during the car ride.